It is very desirable in many instances to detect cracking or rupturing in various structures to avoid damage to the structures and even loss of life or limb due to failure of such structures. Many different systems have been devised to monitor and detect cracks or other defects in structures to which they are applied.
Prior art systems have included indicating devices consisting of electrical conductors in the form of wires or thin metallic ribbons of conducting metals bonded to an insulating substrate. The substrate with the bonded conductors may be wrapped around or cemented flat to areas that may rupture from strain or stress. An example of such a system is disclosed in the patent to Dragoumis, U.S. Pat. No. 3,721,898. However, in these crack or rupture-indicating systems the rupture-sensing solid metal conductors themselves can crack either from fatigue or stress even before the monitored structure itself, to which the substrate is applied. If the solid conductors are made of ductile metals, so that they are not too brittle as to break under small elongations, they will usually then have low fatugue life, but even worse, will stretch and span the crack. The reverse is also generally true since high strength of the solid metal rupture-sensing conductors usually means low elongation thereof under stress which will render the rupture-sensing conductors unable to withstand elongations normally expected of the structures to which the substrate is applied. Even though some suitable combinations can be found for some applications, this type of solid metal rupture-sensing conductor bonded to a substrate cannot be used in many applications where the monitored structures have complex uneven surfaces sometimes having multiple curvatures.
Another prior art approach to the crack-detecting problem is illustrated in the patent to Crites U.S. Pat. No. 3,803,485. This patent discloses a system which uses microencapsulated conducting liquids embedded in a coating that can be applied to the surface of the structure to be protected, and can be hardened in place, with a top covering layer of conducting material. This system works very successfully in some applications but is difficult to apply to the structure without rupturing one or more of the capsules. Also, if too much pressure is applied to the coating or the coating is struck with a sufficient blow, a rupture of capsules will take place, indicating falsely that a crack or rupture has occurred in the structure being monitored. Moreover, at extremely low temperatures, the conducting liquids in the capsules do not really flow so as to allow the system to function as intended.
Other prior art rupture-detecting systems involve the use of special metallic grids that are usually photo-etched on a substrate to be applied to the structure to be monitored but these are usually quite expensive. Etched metal film, metal foil and electrically or vapor-deposited metal lines have been used but are difficult and expensive to use. These materials can also fatigue along with the structures to which they are applied, or even before, giving incorrect warning information.
The rupture-detecting system disclosed herein overcomes all of the problems of the above-mentioned prior art systems and others and even accomplishes this with a great reduction in cost of manufacture and with ease of application in an effective manner.